Quote:
Originally Posted by strat61caster
the temperature increases lets say 3 psi after an autox run or lapping session. By using dry air (typically accomplished with nitrogen) you can reduce the pressure change, lets just make stuff up and say it reduces the increase by 1 psi.
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OK, here is an attempt at estimating these effects.
We want to estimate how different is the response of tire pressure to temperature changes depending on the gas used to fill the tire.
You are guesstimating that using regular moist air will cause the response to be 150% compared to dry air or pure nitrogen (3 psi vs. 2 psi).
Now we all now that PV=nRT.
So the response of pressure to temperature change will depend on nR/V. V is constant for our purpose (there may be changes of tire volume with pressure and temperature, but they are minute and more importantly will not differ between different gases). Therefore the response will depend on nR only.
n is the amount of gas in moles, so for a given volume it will be determined by the gas molar volume, which only slightly between water vapor (24.5), oxygen and nitrogen (both 22.4)
For R the difference is bigger, 0.297 for O2, 0.26 for N2, H2O vapor 0.462.
(all figures taken form the internets ;-)
Now I am making a few assumptions here, which may be not generally correct, but I think they are close enough to correct for our purposes. If there is a physicist in the room, please stand up and correct me.
First, I am assuming that n and R change with temperature and pressure will be either negligible or its difference between our gases will be tiny.
Second, I am assuming that n and R of a mixture of gases can be approximated by the weighted average of n & R of component gases.
Third, I am assuming that dry air is 79% nitrogen and 21% oxygen, and that's it.
Fourth, I am assuming that moist air is 5% water vapor + 95% dry air. This is a worst case scenario, there is this much water vapor in the air at 100% relaive humidity at 40C (104F). Normally,at lower temperatures and lower RH there will be much less.
Fifth, I may have mixed up volumes and masses somewhere, but I think these are similar enough between our gases to ignore this.
Now, to the results. nR for dry air is 6.0, for nitrogen 5.82, for water vapor 11.32. this would indicate a huge difference in response of pressure to temperature change, with water vapor's response about 189% of dry air response, and about 194% of pure nitrogen response.
BUT let's not forget that we do not have pure water vapor in the tires, we have 5% max, usually much less.
For 5% water vapor in air my figures are 104% compared to dry air and 107% compared to nitrogen. This is FAR less than 150%, and once again, I assumed using insanely tropical air, normally the effect will be much smaller.